Electric contact



pr i3, 1948. V|. R. HENSEI. ET Al. 2,439,570

ELECTRIC CONTACT Filed NOV. 10, 1942 /TTORNEYS Patented Apr. 13, 1948UNITED STATES PATENT- OFFICE ELECTRIC CONTACT Franz R. Hensel, Earl I.Larsen, and Earl F.A

Swazy, Indianapolis. Ind.. assignox-s to P. R. Mallory & Co., Inc.,Indianapolis. Ind., a corporation of Delaware i Application November1o,194z,seriai No. 465,100 sclaims. (ci. 'z5- 136) sistant electriccontact.

Other objects of the invention will be apparent l from the descriptionand claims.

In the drawings Figure 1 is a face view of an electric contact embodyingfeatures of the invention;

Figure 2 is a side view of the contact and the backing therefor;

Figures 3 and 4 are face and side views, respectively, of a contactsuited for resistance welding to a backing;

Figures 5 and 6 show another contact suitable for resistance orprojection welding;

Figures 7, 8 and 9 are face view, side view and section of another formof contact suited for resistance welding to a backing; and

Figures 10 and 11 are end and side views of a screw type contactassembly.

Recently some uses for electric contacts have been developed wherecontact materials of the prior art have failed to performsatisfactorily. One application has arisen in connection with airplaneengines where the engine vibrations produce a constant scrubbing" orrotary action of the contacts against each other while they areI Theabrasive action accompanying the closed. mechanical rubbing between thecontacts when they are closed appears to grind olf some of the contactmetal at the surfaces in a powder form. This powder, perhaps aided insome cases by the presence of oil or moisture, produces a highresistance material betweenthe contacts and raises the contactresistance toan excessively high value in a relatively short time.Tungsten contacts have been tried for this use but have proven Accordingto the present invention.- the 'material forming the electric contactswill fall within the following range of contact compositions:

. Per cent Refractory metal carbide selected from the group consistingof tungsten carbide molybdenum carbide, titanium carbide, zirconiumcarbide, vanadium carbide.

columbium carbide and tantalum carbide '10 to 98 Cobalt 0.25 to 20Silver 0.25 to 24.9

The term, "silver is intended to include pure silver and high silveralloys, such as coin silver and sterling silver. The principal additivemetals in the case of silver base alloys are copper, gold, silicon,nickel, manganese, tin, indium, platinum and palladium.

The preferred refractory carbide is tungsten carbide. Either one or bothof the tungsten carbides may be used, the preferred one being WC.

v'ithin the above permissible range of compositions, the followingpreferred ranges may be mentioned:

The preferred contact composition, particu1arly for applications onengines where vibration is severe, is:

Per cent (1) Tungsten carbide Cobalt g 8 Silver 2 Other specificcompositions which may be men.- tioned are:

The preferred method of making the contacts comprises mixing the metalpowders together, pressing the powder mixture to form a pressed metalbody and then sintering the pressed powder body at an elevatedtemperature. All the powders used are preferably of -325 mesh iinenessand may, for example, have the following average particle size:

, f' Microns Tungsten carbide 1.5 Cobalt 3 to 5 Silver It is preferredto mix the tungsten carbide, or other refractory carbide and cobalt-powders together first and ball mill them for a considerable period oftime, such as 48 hours. This promotes the production of cobalt envelopessurrounding the tungsten carbide particles. 'I'he silver powder may thenbe added and the mixture ball milled again for several more hours, suchas 12 hours. It is advisable to add a small amount of a volatile bindersuch as 1 or 2% glyptal at this stage to promote bonding of the powdermixture. The mixture may then be pressed in a suitable die to form apressed powder body. A suitable pressed density is 8 grams per c. c. forthe preferred composition (l) given above.

The pressed body may then'be presintered in hydrogen at 850 C. for onehour and then cut or dressed to suitable dimensions, allowing forfurther shrinkage to take place on further sintering. A nal sintering inpure dry hydrogen at a temperature between 1350 C. and 1500 C. is thengiven to the body in a closed carbon boat or tube. The preferredsintering temperature is 1450 C. The sintered pellets of composition (1)may have a density around 13.9 grams per c. c. and a. hardnessaround'85-90 Rockwell A.

It will be appreciated that these physical properties are given by wayof example only and that the properties of the finished product may bevaried depending on the materials and procedures used.

For compositions where the higher silver con- 4 silver to impregnate theremaining pores with silver.

Where operation at extreme temperatures must be provided for. it is ofsome considerable importance to attach the refractory carbide contact toa .backing having substantially the same low co-etlicient of expansionover a comparatively tents are used, it may be possible in some cases topress and sinter the mixture of refractory then to heat the body incontact with molten wide range of temperatures.

Materials of low co-emcient of expansion which are suitable are Invar,30 to 45% nickel steels, alloys such as Kovar or Fernico containing 28%nickel, 18% cobalt, balan'ce iron; materials such as Fernichromecontaining 37% iron. 30% nickel, 25% cobalt and 8% chromium, alsostraight ironchromium alloys.

Referring to the drawing, Figures V1 and 2 show a contact assemblycomprising a facing disc l0 formed of the sintered refractory carbidecomposition described herein attached to an Invar or other low expansionalloy backing rivet lI2 by a thin layer Il of brazing solder, such assilver solder. pure copper or a copper-nickel alloy. The backing rivethas a shank portion I3 to enable it to be attached to a support intheelectrical apparatus in which the contact is used.l

Figures 3 and 4 show a button-type contact having a plane facing disc 2lsecured by braze 2i to a thin nat disc 22 of backing metal. such assteel, Monel, nickel. bronze or nickel-silver or. where thermal stressesare to be kept low, of Invar or other low expansion alloy. Disc 22 isprovided on its back face with a small central rounded dome projection23 which enables it to be resistance welded to a support such as a leafspring or other contact arm by clamping the contact and arm betweenwelding electrodes and passing electric current through thejunction tomelt the projection and cause welding to the contact arm.

Figures 5 and 6 show another contact button .wherein the facing disc Ilof the refractory carbide composition has a rounded face.

Figures '1, 8 and 9 show a contact adapted for resistance weldingcomprising a refractory facing disc 40 having a plane contact face and aconcave under side. Backing l2 is a concave-convex disc, the convex sideof-which lits intov the concave side of disc 4l and is joined to it bybrazing solder Il. The contact can be secured to a support by resistancewelding wherein rim 43 around the concave depression is backing l2 isfused to the support. y

Figures 10 and 11 show a screw type contact and the use of a stressequalizing layer to isolate expansion stresses in the screw backing fromthe contact face. YThe assembly comprises refractory composition contactface disc 5l secured by a braze layer 5| to a stress equilizing disc 52which may be of a, non-brittle lowl expansion alloy, such-as Invar orone of the other alloys mentioned. Disc 52 is in turn secured by brazing'solder layer 53 to the backing screw 5I which may be of relatively highexpansion metal such as steel. brass, bronze or the like. Disc I2prevents the expansion stresses from theibacking screw lfrom beingtransmitted to the relatively brittle contact facing disc.

Instead of making disc 52 of low expansion alloy, it may be formed of ahighly ductile metal or alloy, such as soft silver or copper, whichyields suiliciently to absorb thermal stresses.

In comparison tests in an accelerated wear test machine, designed toproduce contact abra.-

'sion, it was found that' the preferred material for the presentinvention formed of tungsten carbide, 8% cobalt and 2% silver, had acontact resistance after 396 hours of operation o! only .12 ohm andshowed a material loss for the moving contact of .0030 gm. and tor thestationary contact of .0003 gm., as compared with a contact resistanceoi 3.15 ohms and a material loss of .0414 gm. for the moving contact and.0348 gm. for the stationary contact both formed of a high comercialgrade of tungsten contact material after only '120 hours of operation.Another high grade tungsten contact material developed a contactresistance of 4.15 ohms and a material loss of .0350 gm. from the movingcontact and .0370 gm. from the stationary contact after 120 hours ofoperation. Of twenty dierent types of contact materials tested none ofthem exhibited the same advantgeous combination of low contactresistance and low material loss of the preferred material of thepresent invention.

While specific embodiments of the invention have been described, it isintended to cover the invention broadly within the spirit and scope ofthe appended claims.

What is claimed is:

1. An electric contact suitable for operation under conditions oi strongvibration formed of a metal composition of 85 to 95% of a refractorymetal carbide selected from the group consisting of tungsten carbide,molybdenum carbide, titanium carbide. zirconium carbide, vanadiumcarbide, oolumbium carbide and tantalum carbide, from 2 to 10% cobaltand 0.5 to 5% silver said contact being characterized by yhigh abrasionresistance and low material loss.

REFERENCES CITED The following references are of record in the ille ofthis patent:

UNITED STATES PATENTS Number Name Date 1,181,742 Coolidge May 2, 19161,979,372 Duhme Nov. 6, 1934 2,049,317 Pinta July 28, 1936 2,281,446Laise Apr. 28. 1942 2,289,708 Jackson July 14, 1942 2,313,070 HenselMar. 9, 1943 FOREIGN PATENTS Number Country Date 345,171 Great BritainMar. 19, 1931

